Conventionally, it has been considered that the oxidation deterioration of lubricating oil is promoted under high temperature conditions. In recent years, however, attention has been paid to the fact that under the conditions that the oil is present together with water content the deterioration is promoted at relatively low temperature also (IGARASHI, Tribologist, vol. 45, No. 11, (2000), pp. 801-808). It appears that the oxidation deterioration of lubricating oil for internal combustion engines at low temperature is caused by the following: water vapor generated by combustion in an engine or water content in air condenses so as to be incorporated into the lubricating oil; when this remains and accumulates in the lubricating oil without evaporating, the oxidation deterioration is promoted; and then water reacts with sulfur compounds, such as sulfur oxides contained in combustion gas or zinc dithiophosphate contained in the lubricating oil, decomposition products thereof, and other compounds so as to generate strong acids such as sulfuric acid. A metal detergent has effect of neutralizing such strong acids, and is used to heighten the high-temperature detergency of lubricating oil. However, when the metal detergent itself is used up for the neutralization, the oxidation deterioration of the lubricating oil is abruptly caused. It is therefore difficult to maintain the initial performance of the lubricating oil for a long term.
Under conditions for driving an ordinary internal combustion engine, the temperature of a lubricating oil inside its crank case becomes high. Thus, water content incorporated into the lubricating oil as described above, evaporates easily, and the base oil itself of the lubricating oil gives a low solubility of water content. Therefore, the content thereof usually becomes about several tens of ppm by mass. However, water content accumulates in the crank case when the following is performed: the driving in the state that the temperature of the oil is low and water-in-oil does not evaporate easily, for example, the driving from the time when the oil is cool to the time when the oil temperature becomes about 100° C. or higher, the driving for a long term at low temperature, for example, at an oil temperature of 80° C. or lower, the driving in which an engine pauses while idling, or the driving where short-distance running is repeated. About recent lubricating oil, for internal combustion engines, into which a large amount of a dispersing agent such as succinimide is incorporated, water content is easily embraced by the lubricating oil. Thus, the water content by percent in the lubricating oil easily becomes higher than conventional lubricating oil. In particular, in the case where an internal combustion engine using, as a fuel, gasoline, LPG, natural gas or the like in which the amount of water vapor generated by combustion is large, water content is easily condensed by vaporization heat is driven under conditions as described above, the following results are obtained when the water content in the used oil is analyzed: the water content is dissolved and contained in the lubricating oil in an amount of at most 200 to 500 ppm by mass, particularly 1000 ppm or more by mass in the case where the engine is a gas engine and, as the case may be, 10000 ppm by mass, or more. In the case of internal combustion engines for ship, such as outboard engines for motorboats, which are driven over water, lubricating oil is exposed to low-temperature (for example, 50 to 70° C.) and high-humidity conditions, and the conditions are severe conditions against the oxidation deterioration of lubricating oil.
The present inventor has found out that: it is possible for a low-sulfur-content lubricating oil composition in which zinc dithiophosphate, which is excellent in antioxidation and wear prevention, is decreased or is not used in order to restrain the consumption of a metal detergent to restrain the consumption of the metal detergent and improve high-temperature detergency, oxidation stability at high temperature, and others; and further when the sulfur content in fuel used in an internal combustion engine is 50 ppm or less by mass, in particular, 10 ppm or less by mass, the amount of sulfur oxide, which originates from the fuel, incorporated into the lubricating oil is remarkably decreased so that the base number retention property and others of the lubricating oil can be made high (Japanese Patent Application No. 2002-015351).
However, even in such a case in which the amount of strong acid, which originates from sulfur, incorporated into a lubricating oil is remarkably decreased, the following problems become evident under conditions, as described above, that a large amount of water content is present: antioxidation performance and others of a metal detergent, in particular, a salicylate detergent superior in high-temperature detergency and high-temperature oxidation stability are markedly hindered and, as the case may be, precipitation is generated. Thus, it has been desired to solve the problems.